Local, mechanical factors are believed to play a central role in the development and progression of knee osteoarthritis (OA). The effect of a varus thrust may be potent. Visualized during the physical examination of gait observation, varus thrust is the bowing-out of the knee laterally [i.e. the abrupt first appearance of varus (bow-leg) or worsening of existing varus] while the limb is bearing weight during ambulation, with return to less varus more neutral when the limb is no longer bearing weight. There is preliminary evidence of the role of another local factor: meniscal tears may predict loss of articular cartilage integrity on MRI in persons with knee OA. The menisci, which play a key shock-absorbing role in the knee, are more likely to be torn or subluxed (extruded) with greater age. As a comprehensive examination of these factors, an ancillary to the Osteoarthritis Initiative (OAI) is proposed. The OAI is a prospective, longitudinal, cohort study of 4800 men and women either already with symptomatic, radiographic knee OA (the progression cohort) or at high risk to develop it (the incidence cohort). The OAI is occurring at 4 clinical sites with a coordinating center. The funded mission of the OAI is to identify and evaluate biomarkers for structural knee OA outcomes; to accelerate this work, all OAI data will be released to the public. Comprehensive knee image acquisition, including state-of-the-art bilateral 3.0T MRI and radiography, occurs in all OAI participants at baseline and annually. The OAI has been designed to include the full spectrum of knee OA disease. All the data in the proposed ancillary will join the OAI public data set at a specified time point. This study will primarily examine: the relationship of baseline varus thrust to worsening, over the following 3 years, of medial tibiofemoral OA, separately in knees without and with OA at baseline; the relationship of medial meniscal tear and subluxation and subsequent medial OA worsening; whether thrust leads to meniscal tear and subluxation; whether thrust and meniscal tears predict physical function decline. Secondarily, we will examine: the effects of valgus thrust and lateral meniscal lesions on lateral OA worsening; risk factors for incident thrust; the relationship of pain and thrust, over time. Adjustment will be made for potential confounders, which, in specific analyses, may include disease severity, disease progression, age, gender, BMI, physical activity, and injury.